Audio Data Formats 2.0(26-30) - USB_AUDIO@WIKI

原文：Audio Devices Rev. 2.0 Spec and Adopters Agreement(ZIP)

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Universal Serial Bus Device Class Definition for Audio Data Formats
Release 2.0 May 31, 2006 26

Offset	Field	Size	Value	Description
4	qNanoSeconds	8	Number	Offset in nanoseconds from the beginning of the audio stream.

Note: Timing information is intrinsically provided by the isochronous data transport mechanism itself (packets are synchronized to the USB SOF and the number of samples per packet is an overall measure of the audio data sampling rate). However, the high resolution presentation timestamp could potentially be used to deliver more accurate instantaneous timing information to the sink or to report a (constant) delay between the moment of transport over the USB and the moment of actual rendition. Care must be taken to ensure that the information contained in the Packet Header is at all times in agreement with the implicit timing information, delivered by the isochronous streaming mechanism.

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Universal Serial Bus Device Class Definition for Audio Data Formats
Release 2.0 May 31, 2006 27

3 Adding New Audio Data Formats

Adding new Audio Data Formats to this specification is achieved by proposing a fully documented Audio Data Format to the Audio Device Class Working Group. Upon acceptance, they will register the new Audio Data Format (attribute a unique bit position in the bmFormats field of the class-specific AS interface descriptor) and update this document accordingly. This process will also guarantee that new releases of generic USB audio drivers will support the newly registered Audio Data Formats.

It is always possible to use vendor-specific definitions if the above procedure is considered unsatisfactory.

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Universal Serial Bus Device Class Definition for Audio Data Formats
Release 2.0 May 31, 2006 28

4 Adding New Side Band Protocols

Adding new Side Band Protocols to this specification is achieved by proposing a fully documented Side Band Protocol to the Audio Device Class Working Group. Upon acceptance, they will register the new Side Band Protocol (attribute a unique SideBandProtocol constant) and update this document accordingly. This process will also guarantee that new releases of generic USB audio drivers will support the newly registered Side Band Protocols.

It is always possible to use vendor-specific definitions if the above procedure is considered unsatisfactory.

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Universal Serial Bus Device Class Definition for Audio Data Formats
Release 2.0 May 31, 2006 29

Appendix A. Additional Audio Device Class Codes

A.1 Format Type Codes

Table A-1: Format Type Codes

Format Type Code	Value
FORMAT_TYPE_UNDEFINED	0x00
FORMAT_TYPE_I	0x01
FORMAT_TYPE_II	0x02
FORMAT_TYPE_III	0x03
FORMAT_TYPE_IV	0x04
EXT_FORMAT_TYPE_I	0x81
EXT_FORMAT_TYPE_II	0x82
EXT_FORMAT_TYPE_III	0x83

A.2 Audio Data Format Bit Allocation in the bmFormats field

A.2.1 Audio Data Format Type I Bit Allocations

Table A-2: Audio Data Format Type I Bit Allocations

Name	bmFormats
PCM	D0
PCM8	D1
IEEE_FLOAT	D2
ALAW	D3
MULAW	D4
Reserved. Must be set to 0.	D30..D5
TYPE_I_RAW_DATA	D31

A.2.2 Audio Data Format Type II Bit Allocations

Table A-3: Audio Data Format Type II Bit Allocations

Name	bmFormats
MPEG	D0

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Universal Serial Bus Device Class Definition for Audio Data Formats
Release 2.0 May 31, 2006 30

Name	bmFormats
AC-3	D1
WMA	D2
DTS	D3
Reserved. Must be set to 0.	D30..D4
TYPE_II_RAW_DATA	D31

A.2.3 Audio Data Format Type III Bit Allocations

Table A-4: Audio Data Format Type III Bit Allocations

Name	bmFormats
IEC61937_AC-3	D0
IEC61937_MPEG-1_Layer1	D1
IEC61937_MPEG-1_Layer2/3 or IEC61937_MPEG-2_NOEXT	D2
IEC61937_MPEG-2_EXT	D3
IEC61937_MPEG-2_AAC_ADTS	D4
IEC61937_MPEG-2_Layer1_LS	D5
IEC61937_MPEG-2_Layer2/3_LS	D6
IEC61937_DTS-I	D7
IEC61937_DTS-II	D8
IEC61937_DTS-III	D9
IEC61937_ATRAC	D10
IEC61937_ATRAC2/3	D11
TYPE_III_WMA	D12
Reserved. Must be set to 0.	D31..D13

A.2.4 Audio Data Format Type IV Bit Allocations

Table A-5: Audio Data Format Type IV Bit Allocations

Name	bmFormats
PCM	D0

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